Knee injuries are an unfortunate byproduct of today's emphasis on sports and physical fitness; however, effective surgical techniques have been developed to repair injuries such as to the anterior cruciate ligament (ACL) and other components of the knee. In addition, many members of our aging population are candidates for total knee replacement surgery because of disease and/or injury. All of these surgical procedures must be followed by a period of rehabilitation in order for recovery to be complete. Furthermore, some injuries to the knee may not require surgery but instead may require an extensive rehabilitation period. Such rehabilitation generally requires that the knee be flexed and the leg be extended such as occurs in normal walking; however, it is frequently undesirable for a recovering patient to bear weight on his leg while rehabilitating his knee. In addition, when a knee has suffered a trauma or other injury, or after surgery, a person often lacks the necessary muscle control, strength or will to flex his knee and straighten his leg. Consequently, there is a need for a rehabilitation device that can be used to mobilize the joint over period of time as a part of the orthopedic care which follows an injury, illness or surgical procedure.
The therapeutic use of an external force to flex and extend the limb to induce motion is referred to as passive motion. The application of continuous passive motion to a joint following a period of immobilization, injury, surgery or the like, has been shown to reduce post-operative pain, decrease the number of adhesions, decrease the amount of atrophy experienced by the surrounding and supporting muscle, promote the speed of recovery, improve the range of motion in a much shorter time, and reduce the risk of deep vein thrombosis and post-traumatic osteopenia. Depending on the nature and severity of the knee injury or the nature and extent of the surgical procedure performed, therapeutic treatment sessions involving continuous passive motion may be carried out on a daily basis for several days or several weeks.
The concept of a therapeutic use of passive and continuous motion is not new, as evidenced by a number of known devices that are designed to impose continuous passive motion on the limb and joint of a patient for such purpose. For example, U.S. Pat. No. 4,492,222 of Hajianpour describes a knee exerciser comprised of a leg support that is hinged at one end to a thigh support and is fixed at its other end to a motor assembly. The other end of the thigh support is pivotally attached to a frame, and the motor assembly is also pivotally attached to the frame. A screw that is threaded into a tubular portion of the leg support is rotated by the motor to drive the device. The Hajianpour device also includes an up/down counter that is arranged to count revolutions of the motor drive shaft via a magnetic sensor. When the count of the counter reaches either the flexion or extension limits, the direction of rotation of the motor is changed. U.S. Pat. No. 4,558,692 of Greiner describes a motor driven leg exerciser having an adjustable leg support, a movable footrest, a motor, and controls for the user or therapist. In operation, the motor drives a chain driven rod back and forth in an arc to move the leg support. As the rod reaches each end of its arc, it activates a directional switch which in turn stops the motor, causes the device to pause for a predetermined period of time, and reverses the direction of the rod. The arcuate movement of the rod causes the leg support to move the patient's leg from an extended position to a bent position.
As the use of therapeutic continuous passive motion (CPM) machines has increased, so too have the number of developments and improvements in the related technology. For example, U.S. Pat. No. 4,798,197 of Nippoldt et al. and U.S. Pat. No. 4,558,692 of Greiner describe various safety features which, upon the occurrence of any of several conditions, will cause the carriage holding the leg to stop and reverse direction; U.S. Pat. No. 4,825,852 of Genovese et al. describes hinges between the upper and lower members of the leg support which are designed to better mimic the motion of the knee joint and thereby increase patient comfort; U.S. Pat. No. 5,255,188 and No. 5,452,205, both of Telepko, describe a universal controller for a CPM device which includes a clock and a liquid crystal display for displaying the accumulated running time for an exercise session; U.S. Pat. No. 5,682,327 of Telepko describes a direct drive CPM device which maintains an approximately constant angular velocity at the knee so as to increase the comfort level of the patient; and U.S. Pat. No. 4,665,899 of Farris et al. describes a CPM device having control means which allow the user or a therapist to change the degree of extension and flexion of the leg, and also having a repetition counter that can count and display the number of flexion repetitions completed. Furthermore, U.S. Pat. No. 4,566,440 of Berner, et al. and U.S. Pat. No. 5,682,327 of Telepko describe continuous passive motion devices which pivot the patient's leg about a virtual axis that is coincident with the hip pivot axis. This helps to avoid placing unnecessary strain on the patients leg or hip joint, and increases the comfort of the patient as treatment is carried out. Finally, U.S. Pat. No. 5,682,327 of Telepko describes a "warm-up" mode of operation by which the range of motion of the device is automatically and gradually increased over a preset period of time at the beginning of a treatment session. U.S. Pat. No. 4,825,852 of Genovese et al. describes a similar "warm-up" feature by which the programmed force and range of motion is automatically reduced somewhat when exercise is restarted after a rest period.
Despite these improvements in CPM technology, conventional CPM devices suffer from several disadvantages. Among these is the fact that conventional CPM machines generally operate at a constant speed throughout a programmed range of motion. However, the inventors of the present invention have learned that therapeutic benefits may be obtained if a greater portion of the treatment time is spent in flexion (or extension) within a critical portion of the range of motion near the flexion (or extension) limit. Therefore, it would also be desirable if a continuous passive motion device could be developed that would concentrate the time spent in a treatment session in flexing the patient's knee or extending his leg through the critical or working portion of the range of motion. It would also be desirable if such a device could be developed that would be relatively simple for a patient to operate and therefore, more likely to be properly used.
Another disadvantage of the constant speed CPM machines is that the carriage holding the patient's leg is rapidly decelerated from the operational speed of the carriage to zero as the carriage reaches its operational extension or flexion limit, and rapidly accelerated from zero to the operational speed in the opposite direction as the carriage moves away from the limit. Such sudden speed and direction changes are uncomfortable for the patient and may impose undesirable stresses on his knee and leg. It would be desirable therefore, if a CPM device could be developed which would allow the carriage to make "soft turns" when changing directions.